Scrub typhus is a life-threatening disease caused Orientia tsutsugamushi, a LPS-negative bacterium that replicates preferentially in endothelial cells (EC) and monocytes. Approximately one million people are infected every year; about one third of world population is at risk of infection. There is no effective vaccine for this infection information on disease pathogenesis is limited. To address these challenges, we have developed mouse models that can mimic pathological features of human scrub typhus. We found that lethal infection was linked to excessive type 1, but deficient type 2, immune responses, which correlated with vascular damage in multiple organs. However, the underlying mechanisms are poorly defined. The goal of this study is to examine the molecular basis of such dysregulation by utilizing the EC-targeting model of O. tsutsugamushi Karp strain (OtK) infection, the most prevalent strain for human infections. Our central hypothesis is that type 1-skewed inflammatory responses and dysregulated angiopoietin (Ang) expression are leading causes for lethal OtK infection. Aim 1 will examine the cellular sources of IFN-? and dual functions of IFN-?-related responses in bacterial clearance versus immunopathogenesis during Orientia infection. The innate vs. adaptive cell sources of IFN-? and its related CXCR3 chemokines in infected liver, lungs and spleen will be examined by using IFN-? reporter mice, as well as mice deficient in RAG2, IFN-? or CXCR3 expression. Mice receiving anti-IFN-? treatment at different stages of disease will help define in vivo function of IFN-? in bacterial clearance vs. immunopathogenesis. Immunological findings from the single-cell and molecular levels will be integrated with those from tissue bacterial load and pathology evaluations. Aim 2 will test the hypothesis that endothelial activation/dysregulation initiated by bacterial infectionis ameliorated by IFN-?-related cytokines/chemokines. Lethally infected mice will be treated with recombinant Ang 1, alone or together with vascular endothelial growth factor (VEGF) or anti-IFN-?, at early and late stages of disease. Similar treatment in IFN-?-/- or CXCR3-/- mice, as well as EC-focused in vitro studies, will help dissect complex interplays between cytokines and vascular responses. This mechanism-focused study endorses synergy among research teams (each with unique expertise); it utilizes the state-of-art ABSL3/BSL3 facilities at UTMB. The long-term goal of this study is to define the pathogenic mechanisms associated with Orientia infection and to utilize this information for the design of control strategies. Discovery of the onet and nature of initial dysfunction will reveal the critical window for immune intervention, as well s for signature immune profiles for disease prognosis. This timely study will have a broad implication for other intracellular pathogens.